Um.

What he said.

Nice tags.

Wish I had played Red Guard.

Thras is a sea-monster: http://youtu.be/vuqBKiKycbA

I can't watch the video, so maybe this is already what you're implying, but...

Sload never stop growing. Maybe when they get old enough they stop caring / being able to move around. And as they become more sedentary, they become used as islands as dirt settles on their bloated backs.

Pretty much. The mystery is, how did Cyrus get Thras to lurch? Once a Sload retires from the world, there's usually no moving them.

Thras was sunk by Bendu Olo, yet it returned? Clearly, the elder Sload were killed, and it took a while for their heirs to grow big enough.

Or maybe they were able to be coerced into submerging and only the coral reefs on the surface were destroyed by the navy.